The mobility of beach fill in front of a seawall on an estuarine shoreline, Cliffwood Beach, New Jersey, USA

Abstract

A field investigation was conducted to identify beach change associated with emplacement of a stone seawall and beach fill designed to protect an eroding cliff near the west end of a meso-tidal estuary. Visual observations of wave processes reveal a mean breaking wave height of 0·2 m at the most exposed monitoring site, a storm-wave height of 0·43 m and a maximum longshore current velocity of 0·55 ms −1. Maximum net erosion at any one monitoring site over 44 months was 26·3 m 3 m −1 of beach, resulting in a rate of erosion of 2·3 m year −1. Up to 6·8 m 3 m −1 of eolian accretion occurred due to the overly wide nourished beach. Local differences in shoreline orientation, sheltering by headlands, position of the seawall on the beach profile, shore-perpendicular structures and sediment availability resulted in variability in beach processes and changes in beach profile. Flanking occurred at the southeast end of the seawall, where fill material bypassed the upper foreshore and moved along the low tide terrace, favored by a pronounced break in shoreline orientation. Net erosion did not occur at the west end of the seawall because sufficient fill sediment was available from updrift sources. Where no fill remained in front of the seawall, wave energy dissipation on the outer low-tide terrace at low tide and wave reflection off the structure without breaking at high tide diminished the effectiveness of wave-energy concentration and turbulence on the low-tide terrace at the base of the wall, and there was no net scour. Shore-parallel structures constructed on the low-tide terrace on meso-tidal estuarine shorelines may not require a fronting protective beach to prevent net toe scour, but nourishment may delay flanking problems on the upper foreshore at downdrift sites.